Explanation
Correct answer: B.
Choice B is the best answer. The underlined sentence provides information about import/export firms, showing how Chinese communities across the world were connected by trade routes.
Why the other choices are wrong
Choice A
Choice A is incorrect. The underlined sentence never suggests that the countries of origin of the fish are in question—in fact, it tells us exactly where they came from.
Choice C
Choice C is incorrect. The passage never describes the steps taken to discover the fish bones described in the previous sentence.
Choice D
Choice D is incorrect. The underlined sentence doesn’t outline a hypothesis but instead provides evidence. And the following sentence agrees with the underlined sentence, so we could eliminate this choice just for saying that the following sentence "casts some doubt on" the underlined one—partly wrong is all wrong.